[gemcoder21]

Solana address history now uses the chain's node client like other node-backed chains: SolanaProvider implements ChainTransactions with getSignaturesForAddress and returns the signatures as transaction requests, which are then fetched by hash from the node as before. The Alchemy-backed SolanaIndexer and its fixture are removed, so Solana no longer uses Alchemy.

• gem_solana: GetSignaturesForAddress on SolanaRpc and SolanaClient::get_signatures_for_address, the TransactionSignature model, map_transaction_requests, ChainTransactions for SolanaProvider in provider/transactions.rs; SolanaProvider::new takes only the client, replacing new_rpc_only
• chain_providers and gemstone: build SolanaProvider from the node client
• docs/FEATURES.md: Solana's history provider is the node

Checks: just lint, clippy on all targets for gem_solana, chain_providers and gemstone, just test for gem_solana, chain_providers and gemstone, and the repo checks pass. builds_signatures_for_address_request pins the request. The gated live test test_solana_get_transactions_by_address needs a node that serves getSignaturesForAddress history; the public node in Settings.yaml returns none, so it fails there.